Valve Operating Systems

In engines with overhead poppet valves (OHV-verhead valves), the camshaft is either mounted in the cylinder block, or in the cylinder head (OHC-overhead camshaft). Figure 2.21a shows an overhead valve engine in which the valves are operated from the camshaft, via cam followers, pushrods, and rocker arms. This is a cheap solution because the drive to the camshaft is simple (either gear or chain), and the machining is in the cylinder block. In a "V" engine, this arrangement is particularly suitable because a single camshaft can be mounted in the valley between the two cylinder banks.

In overhead camshaft (OHC) engines (Fig. 2.2 lb), the camshaft can be mounted either directly over the valve stems, or it can be offset. When the camshaft is offset, the valves are operated by rockers, and the valve clearances can be adjusted by altering the pivot height or, as in the case of the exhaust valves in Fig. 2.21 b, different thickness shims can be used. For the inlet valves in Fig. 2.21b, the cam operates on a follower or "bucket." The clearance between the follower and the valve end is adjusted by a shim. Although this adjustment is more difficult than in systems using rockers, it is much less prone to change. The spring retainer is connected to the valve spindle by a tapered split collet. The valve guide is a press-fit into the cylinder head, so that it can be replaced when worn. Valve seat inserts are used, especially in engines with aluminum alloy cylinder heads, to ensure minimal wear. Normally, poppet valves rotate to even out any wear and to maintain good seating. This rotation can be promoted if the center of the cam is offset from the valve axis. Invariably, oil seals are placed at the top of the

valve guide to restrict the flow of oil into the cylinder. This is most significant with overhead cast-iron camshafts, which require a copious supply of lubricant. When the valves are not in line (b), it is more usual to use two camshafts because this gives more flexibility on valve timing and greater control if a variable valve timing system is to be used.

The use of four valves per combustion chamber is quite common in high-performance spark ignition engines and is used increasingly in compression ignition engines. The advantages of four valves per combustion chamber are larger valve throat areas for gas flow, smaller valve forces, and a larger valve seat area. Smaller valve forces occur because a lighter valve with a less stiff spring can be used. This also will reduce the hammering effect on the valve seat when the valve closes. The larger valve seat area is important because this is how heat is transferred (intermittently) from the valve head to the cylinder head. In the case of diesel engines, four valves per cylinder allow the injector to be placed in the center of the combustion chamber, which facilitates the development of low-emission combustion systems.

To reduce maintenance requirements, it is now common to use some form of hydraulic lash adjuster (also known as a hydraulic lifter or tappet), an example of which is shown in Fig This consists of a pistonlcylinder arrangement that is pressurized by engine lubricant. However, when the cam starts to displace its follower, a sudden rise in pressure occurs in the lower oil chamber. This causes a check valve (a ball loaded by a weak spring) to close, so that the cam motion then is transmitted to the valve. There is always a small leakage flow from the lash adjuster so that the valve will always seat properly, even when there is a reduction

in the clearances within the valvetrain. The lash adjusters can be incorporated into the follower of the overhead valve arrangement (Fig.a) or the bucket tappet of Fig. b, or the pivot post of a cam-over-rocker system. A disadvantage of this simple substitution is an increase in frictional losses because the cam follower will always be loaded when sliding on the cam base circle. Friction can be reduced by using a roller follower on the rocker of the system in Fig. 2.22, and this cam-over-rocker system also minimizes the mass of the moving valvetrain components. A hydraulic lash adjuster reduces the stiffness of the valvetrain, which will reduce the maximum speed limit for the valve gear.

The drive to the camshaft usually is by chain or toothed belt. Gear drives also are possible but tend to be expensive, noisy, and cumbersome with overhead camshafts. The advantage of a toothed belt drive is that it can be mounted externally to the engine, and the rubber damps out torsional vibrations that otherwise might be troublesome.

Antioxidants are needed in gasoline to inhibit the formation of gum, which usually is associated with the unsaturated hydrocarbons in fuel. Formation of gum can interfere with the operation of fuel injectors.

Detergents are added to reduce the deposits in fuel injectors, the inlet manifold, and the combustion chamber. Surfactants inhibit the formation of deposits in the injectors and the inlet manifold, but a different mechanism is needed to combat valve and port deposits because these deposits are associated with higher temperatures. High-boiling point, thermally stable, oily materials such as polybutene are used, and these appear to dissolve the deposits. 49 Diesel additives to improve the cetane number will be discussed first, followed by additives to lower the cold filter plugging point temperature, then additives that are used with low sulfur fuels, and finally other additives.

The most widely used ignition-improving additive currently is 2-ethyl hexyl nitrate (2EHN), because of its good response in a wide range of fuels and comparatively low cost (Thompson et al., 1997). Adding 1000 ppm of 2EHN will increase the cetane rating by approximately 5 units. In some parts of the world, legislation limits the nitrogen content of diesel fuels, because although the mass of nitrogen is negligible to that available from the air, fuel-bound nitrogen contributes disproportionately to nitric oxide formation. Under these circumstances, peroxides can be used, such as ditertiary butyl peroxide (Nandi and Jacobs, 1995).

Diesel fuel contains molecules with approximately 12 to 22 carbon atoms, and many of the higher molar mass components (e.g., cetane, C16H34) would be solid at room temperature if they were not mixed with other hydrocarbons. Thus, when diesel fuel is cooled, a point will be reached at which the higher molar mass components will start to solidify and form a waxy precipitate. As little as 2% wax out of the solution can be enough to gel the remaining 98%. This will affect the pouring properties and (more seriously at a slightly higher temperature) block the filter in the fuel-injection system. These and other related low-temperature issues are discussed comprehensively by Owen and Coley (1995), who point out that as much as 20% of the diesel fuel can consist of higher molar mass alkanes. It would be undesirable to remove these alkanes because they have higher cetane ratings than many of the other components. Instead, use is made of anti-waxing additives that modify the shape of the wax crystals.

Wax crystals tend to form as thin "plates" that can overlap and interlock. Anti-waxing additives do not prevent wax formation. They work by modifying the wax crystal shape to a dendritic (needle-like) form, and this reduces the tendency for the wax crystals to interlock. The crystals are still collected on the outside of the filter, but they do not block the passage of the liquid fuel. The anti-waxing additives in commercial use are copolymers of ethylene and vinyl acetate, or other alkene-ester copolymers. The performance of these additives varies with different fuels, and the improvement decreases as the dosage rate is increased. It is possible for 200 ppm of additive to reduce the cold filter plugging point (CFPP) temperature by approximately 10 K.

Additives can be used with low-sulfur diesel fuels to compensate for their lower lubricity, lower electrical conductivity, and reduced stability. To restore the lubricity of a low-sulfur fuel to that of a fuel with 0.2% sulfur by mass, then a dosage on the order of 100 mg/L is needed. Care is required in the selection of the additive, if it is not to interact unfavorably with other additives (Batt et al., 1996).

Electrical conductivity usually is not subject to legislation, but if fuels have a very low conductivity, then there is the risk of a static electrical charge being built up. If a road tanker, previously filled with gasoline, is being filled with diesel, then there is the possibility of a flammable mixture being formed. The conductivity of untreated low-sulfur diesel fuels can be less than 5 pS/m (Merchant et al., 1997). Conductivities greater than 100 pS/m can be obtained by adding a few parts per million of a chromium-based static dispersant additive. Low-sulfur fuels and fuels that have been hydro-treated to reduce the aromatic content also are prone to the formation of hydroperoxides. These are known to degrade neoprene and nitrile rubbers, but this can be prevented by using antioxidants such as phenylenediamines (suitable only in low-sulfur fuels) or hindered phenols (Owen and Coley, 1995).

Other additives used in diesel fuels are detergents, anti-ices, biocides, and anti-foamants.

Detergents (e.g., amines and amides) are used to inhibit the formation of combustion deposits. Most significant are deposits around the injector nozzles, which interfere with the spray formation. Deposits then can lead to poor air-fuel mixing and particulate emissions. A typical dosage level is 100-200 ppm.

Anti-ices (e.g., alcohols or glycols) have a high affinity for water and are soluble in diesel fuel. Water is present through contamination and as a consequence of humid air above the fuel in vented tanks being cooled below its dewpoint temperature. If ice formed, it could block both fuel pipes and filters. Biocides act against anaerobic bacteria that can form growths at the wateddiesel interface in storage tanks. These are capable of blocking fuel filters.

Anti-foamants (1 0-20 ppm silicone-based compounds) facilitate the rapid and complete filling of vehicle fuel tanks.